The present invention relates to signal processing, particularly in the fields of wireless ATM and fixed-radio access to the Internet. More specifically, it pertains to a method and apparatus for performing image signal rejection.
Based on the proliferation of high-speed wireline Internet access at 2-10 Mb/s, it is obvious that there will be future demand for wireless access at comparable rates. Unlike the crowded 2 GHz band, the 5 GHz bands, with more than 300 MHz available, appear extremely attractive for high-speed applications such as wireless ATM and wideband fixed-radio access. In such applications, the design of a radio transceiver (transmitter and receiver) is critical to the successful realization of increased wireless access rates.
Standard practice in Radio Frequency (RF) receiver design has been to convert high frequency RF signals to one or more Intermediate Frequencies (IF) which, in a final translation step, are converted to baseband or Direct Current (DC). Such IF signals are by nature at a lower frequency than the RF signals. The opposite conversion is effected at the transmitter. Each translation stage normally uses a mixer and an image rejection filter, where each mixing operation produces both the desired signal and an image signal. The image signal falls into the same IF frequency band as the desired signal and corrupts it, and must be filtered out. An Image Rejection Filter (IRF) is thus required in order to suppress the image signal.
In existing radio transceivers that are implemented on a chip, off-chip fixed filters are used to achieve image signal rejection. Unfortunately, such filters require additional pins and interfaces, thus increasing power requirements, package complexity and cost. Although tunable on-chip bandpass filters have been considered, they have generally been found to be unfeasible as it is very difficult to obtain stability and accuracy in the tune frequency.
In an alternative, image reject mixers have been used in prior art designs, whereby two mixers are driven by Local Oscillator (LO) voltages shifted by 90 degrees, capable to selectively favor the desired RF signal over the image signal. In practice, these image reject mixers are capable of a certain amount of image rejection, however it is generally limited to less than xe2x88x9240 dB. Moreover, such image reject mixers require extra circuitry, resulting in increased power consumption, noise and chip area.
The background information provided above shows that there exists a need in the industry to provide an improved method and apparatus for performing image signal rejection.
The present invention is directed to a method and apparatus for achieving tunable image signal rejection in a radio signal processing device. The radio signal processing device includes a tunable local oscillator to generate a frequency signal at a selected frequency within a certain frequency range, and a receiver for receiving an RF signal, where the receiver itself includes a mixer and a filter. The mixer is responsible for converting the RF signal to an IF signal. The filter is coupled to the mixer for performing image signal rejection at an image reject frequency, and is designed to alter the image reject frequency such that the image reject frequency tracks the local oscillator frequency.
By incorporating the image rejection filter into the receiver such that the device can be implemented on a single chip, and by ensuring that the image reject frequency of the filter tracks the local oscillator frequency, stable on-chip tuning of the image rejection filter can be achieved. The stability of this design provides an important advantage over prior art designs of on-chip image reject filters, in which it is difficult to achieve stability and accuracy. Further advantages include a considerable cost saving and a reduction in the chip area.
In a specific example of implementation, the radio signal processing device includes a frequency control unit. This frequency control unit generates a control signal for tuning the local oscillator to a particular frequency. Specific to the present invention, the control signal is used to simultaneously tune both the local oscillator and the receiver filter to their respective frequency of operation.
In another example of implementation, the radio signal processing device is a radio transceiver which additionally includes a transmitter for transmitting an RF signal. The transmitter includes a power amplifier, an amplifier driver, a mixer and a filter. The mixer is responsible for converting an IF signal to an RF signal. The filter is coupled to the amplifier driver for performing image signal rejection at an image reject frequency, and is designed to alter the image reject frequency such that the image reject frequency tracks the local oscillator frequency. The control signal generated by the frequency control unit is used to simultaneously tune the local oscillator, the receiver filter and the transmitter filter to their respective frequency of operation.
The present invention also provides a mixer and filter combination for achieving image signal rejection.
The present invention also relates to a method for rejecting an image signal resulting from the conversion of an RF signal to an IF signal in a radio signal processing device.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.